After 35 minutes, he started charging his phone. 21 minutes later, the battery is at 23%. Explain how you would determine when the phone will be charged to 100%

91.30 minutes

Step-by-step explanation:

Assuming that when you start charging the phone it had 0% charge, what you should do is a rule of three.

That is, yes in 21 minutes I charge 23%, how long will it take to charge 100%.

charge time     charge percentage.

21 minutes                  23%

 x minutes                  100%

Then it would be:

x = 100 * 21/23 = 91.30

So you would have to wait 91.30 minutes to wait for the full charge.